Step 1: Determine the zip code of the site under consideration. STAAD will automatically find the latitude and longitude of the location, thereby, deriving the Ss and S1 values

Alternatively, if zip code is unknown, then the latitude and longitude may be provided.

You may also manually input the value of Ss and S1 mentioned in equation 11.4.1 of ASCE 7-05 by calculating the same.

Step 2: STAAD determine Ss for site class B (if the same has not been input manually) , you will find this mentioned in equation 11.4.1 of ASCE 7-05.

Step 3: STAAD determines S1 for site class B.( if the same has not been input manually) You will find this mentioned in equation 11.4.1 of ASCE 7-05.

Note: 11.4.1 : Ss and S1:

Ss = mapped MCE,(Maximum Considered Earthquake) 5 percent damped spectral acceleration parameter at short period as defined in section 11.4.1. The parameter Ss shall be determined from the 0.2 s spectral response accelerations shown on Figs. 22-1 through 22-14, respectively.

S1 = mapped MCE,(Maximum Considered Earthquake) 5 percent damped spectral acceleration parameter at short period as defined in section 11.4.1. The parameter S1 shall be determined from the 1 s spectral response accelerations shown on Figs. 22-1 through 22-14, respectively.

Where S1, is less than or equal to 0.04 and Ss is less than or equal to 0.15, the structure is permitted to be assigned to Seismic Design Category A and is only required to comply with Section 11.7.

Step 4: Determine the site class. Site is classified as Class A, B, C, D, E and F depending on the type of soil strata. It is identified as 1 to 6 in STAAD respectively. It is explained in section 20.2, 20.3 and Table 20.3-1.

Step 5: The Ss and S1 values obtained in step 3 are for site class B. If the actual site class, as obtained in step 2, is something besides "B", then, an adjustment needs to be made. This is what leads to the calculation of Sms and Sm1 as mentioned in equations 11.4-1 and 11.4-2 of ASCE 7-05, and, equations 16-37 and 16-38 of IBC 2006. To do this, one needs Fa and Fv. Fa and Fv are obtained from table 1613.5.3(1) and 1613.5.3(2) of IBC 2006, and, table 11.4-1and 11.4-2 of ASCE 7-05.

Alternatively, go to Table 1604.5 of IBC 2006 and find the Occupancy Importance factor ‘I'. Specify this ‘I' in the STAAD input file under the DEFINE IBC command

Step 9: Determine RX and RZ (Response Modification factors) from Table 12.2-1and 12.14-1 of ASCE 7-05 for building structures. It depends on type of resisting structures. It is also defined in Table 15.4-1 and 15.4-2 for non -building structure types. These are used in sections 12.8.1.1, equations (12.8-2), (12.8-3), (12.8-4) and (12.8-6) of ASCE 7-05 Specify RX and RZ in the STAAD input file.

Step 10: Specify the site class as determined from step 4. While the code uses the terms A through F, STAAD uses 1 through 6 as explained earlier in a table this document

Step 13: Determine Ct and x. Ct is building period coefficient and x is the exponent used in the calculation of approximate time period by eqn12.8-7. These two parameters depend on the type of structure and are explained in Table 12.8-2. Specify Ct in the STAAD input file. In case no value is input, then STAAD goes with the default value of 0.03 Ct and 0.75 for ‘x'

Note: It is correct that the Ct value and x are dependent on the structure type. So for these values to be considered correctly it is necessary for the software to understand whether the frame is a moment frame or a braced frame or whether it is completely restrained against lateral movement ( non sway type ) or free to move laterally or whether it is made of steel, concrete or any other material. Whereas it is possible for the software to figure out the material, currently there is no way to input the other information. So based on the material it assumes a reasonable value for Ct ( 0.02 , 0.03 etc ). It is because of this limitation that we provide the users with the option to define terms like Ct, Px, Pz etc. Ct when specified is used in computation of Ta. Moreover the user can directly specify Px and Pz which if specified are considered as fundamental periods in the respective directions. So if the user is not satisfied with the values calculated by STAAD, he/she has the option to change it as per requirement

Step 14: If you wish to specify your own periods to override the values that STAAD calculates using Rayleigh's equations, specify them using the terms PX and PZ.

Step 15: If you wish to include ACCIDENTAL Torsion as described in section 12.8.4.2 of ASCE 7-05 for IBC 2006, specify the keyword ACCIDENTAL in the DEFINE IBC 2003 command.

If the ACCIDENTAL option is specified, the program calculates the additional torsional moment. The lever arm for calculating the torsional moment is obtained as 5% of the building dimension at each floor level perpendicular to the direction of the IBC load (section 12.8.4.2 of ASCE 7-05 for IBC 2006). At each joint where a weight is located, the lateral seismic force acting at that joint is multiplied by this lever arm to obtain the torsional moment at that joint.